1. Field of the Invention
The present invention relates to a force sensor for detecting a force, and more particularly, to a piezoelectric vibration type force unit, a piezoelectric vibration type force sensor, and a robot apparatus, in which a resonance phenomenon of a piezoelectric material is utilized to detect a force applied to the piezoelectric material.
2. Description of the Related Art
Conventionally, there are known various force sensors using a strain gauge, a capacitance variation, conductive rubber, and the like. Those force sensors all utilize a deformation of a member constituting the force sensor (for example, a metal resistor in a strain gauge). Therefore, in order to realize high sensitivity and a wide range of measurement, sufficient deformation of the member is required. For this reason, there is a problem that when the sensor is downsized, sufficient deformation cannot be obtained in some cases so that accuracy may be reduced because an output signal is lost in noise.
In contrast, there is disclosed a piezoelectric vibration type force sensor using a resonance phenomenon of a piezoelectric material having a property of vibrating in a specific direction along with variation with time of a voltage amplitude when an AC voltage is applied (see Japanese Patent Application Laid-Open No. S52-78473).
As described in Japanese Patent Application Laid-Open No. S52-78473, the piezoelectric vibration type force sensor includes a piezoelectric material to which an impedance element is connected in series, and a voltage of a frequency not higher than a resonance frequency of the piezoelectric material is applied for exciting the piezoelectric material. In this state, the frequency of vibration is fixed while an external force is applied to the piezoelectric material. Then, the impedance element of the piezoelectric material is increased so that the resonance frequency is moved to the high frequency side. When an external force is applied to the piezoelectric material in the state in which the resonance frequency is moved to the high frequency side, a value of the voltage applied to the piezoelectric material changes from the value before the external force is applied. Based on this change of the voltage value, the external force applied to the piezoelectric material can be calculated in a quantitative manner. Because the piezoelectric vibration type force sensor has a large change of the voltage amplitude under a small load, the piezoelectric vibration type force sensor has an advantage of high detection sensitivity (see FIG. 4 to be described later).
Now, through different from the piezoelectric vibration type force sensor, for reference, a method of mounting a piezoelectric material onto a piezoelectric force sensor is described with reference to Japanese Patent Application Laid-Open No. H01-260334. Japanese Patent Application Laid-Open No. H01-260334 discloses a structure of a piezoelectric force sensor that detects contact pressure distribution based on voltages generated from multiple piezoelectric materials (piezoelectric elements) arranged in matrix. The piezoelectric force sensor described in Japanese Patent Application Laid-Open No. H01-260334 includes a pressure plate for detecting a force, which is disposed on the side to which the force is applied, and a pressure plate disposed above the piezoelectric material so that the pressure plate can make press-contact with the upper end of the piezoelectric material. Therefore, for example, if a defect occurs in a part of the multiple piezoelectric materials, the piezoelectric material can be accessed easily by removing the pressure plate.
Incidentally, the piezoelectric vibration type force sensor is used in a state in which the piezoelectric material is vibrated by applying a voltage. Therefore, it is necessary to adopt a mounting structure for applying an AC voltage to a drive electrode attached to the piezoelectric material in a state in which the piezoelectric material can vibrate. For instance, it is necessary to adopt a mounting structure for supplying electric power to the piezoelectric material while holding the vibrating piezoelectric material at a predetermined position.
In this regard, for example, it is conceivable to fix the piezoelectric material with a fixing attachment or an adhesive as in the piezoelectric force sensor disclosed in Japanese Patent Application Laid-Open No. H01-260334. However, the piezoelectric vibration type force sensor is used in a state in which an AC voltage is applied to the piezoelectric material to vibrate, and hence if the piezoelectric material is fixed with a member having high stiffness in the vibration direction, or if a frictional surface of the piezoelectric material is fixed with an adhesive, vibration of the piezoelectric material is suppressed. Therefore, in a mounting structure in which the piezoelectric material is fixed to suppress the vibration, a detected value for a change of small vibration amplitude is decreased so that sensitivity is lowered. In addition, if a vibration amount is changed depending on a fixing manner with a fixing attachment or an adhesive despite the same AC voltage is applied to vibrate, a value of the sensor becomes unstable.